The present invention relates to a heat-roller, a fixing means using this heat-roller, and to an image forming apparatus using this fixing means.
Recently, low power consumption is being demanded in image forming apparatus such as copiers, laser printers, etc. In these image forming apparatus, the section in which electric power is most consumed, is the fixing means by which toner on a recording sheet, onto which a toner image has been transferred, is fused and the toner image is fixed onto the recording sheet. Further, it is a heat-roller which is a heat source in the fixing means.
FIG. 5 is a structural view of a heat-roller disclosed in Japanese Patent Publication Open to Publication Inspection No. 55368/1985. FIG. 5(a) is a left end view, FIG. 5(b) is a frontal cross-sectional view, and FIG. 5(c) is a right end view. In FIG. 5, numeral 1 is a heat-roller, and numeral 2 is a recording sheet, wherein one side of the recording sheet is aligned with the reference position, the recording sheet is fed to the heat-roller 1, and a toner image is transferred onto the recording sheet.
A heater lamp 3, which is a heat source, is provided inside the heat-roller 1. Around the heater lamp 3, a plurality of heat pipes 4 (6 pipes in this example), by which heat is transmitted in the axial direction, are provided at equal angles.
Since this heat pipe 4 is expensive, the pipes 4 are not provided through the entire length of the heat-roller 1. The heat pipe 4 is provided as follows. The center (o') of the effective length in the axial direction of the heat pipe 4 is positioned on the side opposite of a reference for sheet passage (ST) with respect to the center (o) of the sheet passage width of the heat-roller 1.
The heat pipe 4 will be described below, while referring to FIG. 6. Each heat pipe 4 comprises: a sealed vacuum container; a porous sheet 6 provided inside the sealed container 5; and a working solution (water, freon, etc.) filling the sealed container 5.
Heat transmission of this heat pipe 4 is carried out as follows.
(1) When a portion of the heat pipe 4 is heated, the working solution is evaporated, and the vapor pressure increases. Thereby, the surface level of the working solution is lower than the porous sheet 6. PA1 (2) The vapor pressure in the evaporation portion is higher than that in the condensation portion, and a pressure difference is caused, and the vapor flows to the condensation portion. PA1 (3) The working solution in the condensation portion is cooled, condensed, and acts as a heat absorption source due to the latent heat of vaporization. PA1 (4) The surface level of the solution in the condensation portion rises, and the surface of the solution tends to remain flat. Thereby, the working solution moves to the heating portion due to capillary phenomenon.
Temperature distribution of the heat-roller 1 when a small sized recording sheet continuously passes over the heat-roller 1, using such a heat pipe 4, is shown in FIG. 7. In this connection, for two cases, one in which the heat pipe 4 is used, and the other when no heat pipe 4 is used, the same electric power is applied.
In FIG. 7, in the case of no heat pipe (plotted by .smallcircle.), the temperature is rapidly lowered toward the sheet passage reference position, which is a positional reference for the conveyance of the recording sheet, due to heat transmission to the recording sheet, and heat radiation of the heat-roller 1 from the edge of the sheet passage reference side.
On the other hand, in the case of the heat-roller using the heat pipe 4 (plotted by .circle-solid.), there is almost no temperature change over the range in which the heat pipe 4 exists. The temperature is lowered from the portion at which the heat pipe 4 does not exist, toward the sheet passage reference position, due to the above-described reasons. However, the amount of temperature decrease is smaller than in the case of no heat pipe.
However, in the heat-roller 1 structured as described above, the heat pipe 4 does not exists in the vicinity of the sheet passage reference side. Accordingly, temperature decrease is inevitable in the vicinity of the sheet passage reference side.
Accordingly, in order to transfer the toner onto the recording sheet, it is necessary to set the temperature (t) at the sheet passage reference position higher than the fixing temperature (T). In this case, the temperature difference between the temperature at other portions, except the sheet passage reference position, and the fixing temperature, becomes large, and the power consumption still remains large, which is a problem.